Aug 23 2020
I just cleaned the browser cookies and it is now working again. Maybe a note about that could be added somewhere on documentation...
Ok. Here is what I have done (a.k.a. steps to reproduce):
Aug 22 2020
Jul 21 2020
Jun 25 2020
Mar 31 2020
It is not present in Page Forms 4.6 (MediaWiki 1.33.0). Not tested in newer versions (but, I believe it is not present either).
Mar 19 2020
May 7 2018
@Kghbln You're right. I was a little bit confused because in 4.3 $wgPageFormsUseDisplayTitle has changed to true. 
So, changing it to false will vanish the warnings (but you will lose the benefits of using DisplayTitle, wich in my case is not desirable).
Just linking to another report of this problem: https://www.mediawiki.org/wiki/Extension_talk:Page_Forms#Warnings
Dec 9 2017
This problem has returned. Tested in PF 4.2, MW 1.30.0